Highlander has existed for ages, allows you to play literally every class, and has 13 banned weapons currently (3 banned for exploits with them) and is so dead it now has less players playing it currently in North America than when I started playing it 7 years ago.
Fact is the competitive scene in totality (HL & 6s) is dying. HL dying is a bad sign because it counters all of your points - which are relatively common points - and yet it can barely bring in players anymore. The top end of HL has withered to the point of near non-existence - there are like...2 actual good teams and they don't really try and still pound everyone else and it bleeds tons of players season to season for the past 3.5 years pretty much.
More over, the actual in-game competitive queue is also pretty dead, which is also a mode where you can play any class and every item is allowed. It hasn't gripped the current player base that well at all.
The problem with TF2 competitively has always been two fold:
the game you pub is not the game you play competitively - this is true even with Valve's current solution. This is pretty unavoidable because most of the player base pubs but "pubbing" is 12v12 which isn't sustainable at all as an e-sport (the LAN costs alone would be ridiculous let alone team issues).
Valve hasn't made a real effort to back any form of competitively monetarily. This is important because without money people eventually lose a reason to play. Most of Highlanders top end bled off either into 6s or other games because you can only earn so many 1st-3rd place medals before you don't really care anymore. Top Invite players in ESEA quit because, well, you could do that or you could go play Overwatch and still have relative amounts of fun with a company that will actually take their feedback and handle its competitive scene well while making actual money that you can live off of.
I've played 6s in the past but it was a long long time ago (2009) and I'm hardly a 6s elitist, but pinning the whole reason 6s is dying on the idea that people can't main Pyro or some weapons are banned is just ignorant. If those were the main reasons then the competitive queue in-game would be loaded with players. It isn't. Highlander would still be huge. It isn't.